1. Field of the Invention
The present invention relates to a multi-layer film substrate and a process for the production thereof. More particularly, the present invention relates to a multi-layer film substrate comprising an insulating layer composed of a polyimide having a low thermal expansion, and a process for the production thereof.
2. Description of the Related Art
In the production of a multi-layer film printed layer, a polyimide is generally used as an insulating interlayer. This polyimide is formed by polymerization of an acid anhydride and a diamine, and a process in which a photosensitive group is given to the polyimide and the polyimide is processed by light exposure and development, a process in which the polyimide in the half-imide state is dissolved in a resist liquid developer, and a process in which a polyimidized film is processed by RIE (reactive ion etching) using a mask of SiO.sub.2 or the like, have been adopted.
Nevertheless, this polyimide has a large thermal expansion, and at the polyimidization termination point, a tensile stress of 3.5 to 4.5 kg/mm.sup.2 is imposed on the substrate, and therefore, the polyimide is defective in that, since the substrate is influenced by the tensile stress whereby a film printed pattern is damaged, the number of layers that can be laminated is limited.
Under these circumstances, there is an urgent need in the art for the provision of a multi-layer film substrate wherein the number of layers can be increased.
As the multi-layer film substrate in which the thermal expansion is reduced and an interfacial peeling is prevented, there has been proposed a multi-layer film substrate in which a polyimide having a small thermal expansion coefficient is used as the insulating layer.
In this multi-layer film substrate comprising an insulating layer of a low thermal expansion type polyimide, since the molecular structure of the polyimide is a rigid linear structure, molecule chains arranged in parallel form mica-like bonds. Therefore, this polyimide is defective in that the adhesion is extremely poor.